In many process engineering facilities, fluids are injected into a gaseous fluid, e.g., into flue gas that is to be cleaned or cooled. In so doing, it is often highly important that the fluid be atomized in the smallest possible droplets. The smaller the droplets, the larger the specific droplet surface. In process engineering, this can result in significant advantages. For example, the size of the reaction container and the costs for its manufacture depend decisively on the average droplet size. However, in many cases it is by no means sufficient if the average particle size drops below a specified limiting value. Only a few substantially larger droplets can result in considerable disruptions of operation. This is the case, in particular, when the droplets—due to their size—do not evaporate rapidly enough, so that the droplets or even pasty particles are deposited in subsequent components, e.g., on fabric filter tubing or on blower blades, thus leading to malfunctions due to incrustations, corrosion or imbalance.
When fluids are to be atomized to form the finest-possible droplet spray, so-called pressurized gas assisted two-substance nozzles are frequently used in addition to high-pressure single-substance nozzles that are loaded only with the fluid that is to be atomized. In these nozzles, the fluid is sprayed with the assistance of a pressurized gas, e.g., pressurized air or pressurized steam, said pressurized gas forming the first gaseous fluid, into a second gaseous fluid, e.g. flue gas.
In order to simplify the language used, the first gaseous fluid is frequently referred to as the “pressurized air”, even though—in generalized terms—it would be possible to refer to pressurized gas or pressurized steam. Further, as a rule, the second gaseous fluid is referred to as the flue gas.
Depending on prior art that exists concerning the respective applications, a multitude of different two-substance nozzles are available. An important criterion considering the field of use is the composition of the fluid that is to be atomized.
1. Nozzles for the Atomization of Fluids that are Free of Solids.
Relatively easy constraints exist only when the fluid does not contain any suspended matter and when the fluid does not form any solid evaporation residues. This applies, e.g., to nozzles for the atomization of ammonia water in systems for the reduction of nitrogen oxide in flue gas, or to nozzles for the atomization of kerosene in turbine jet engines. In particular, for the last-mentioned case of use, so-called prefilming nozzles were developed, such as are shown in FIG. 1. FIG. 1 has been taken from Joos, F., Simon, B., Glaeser, B., Donnerhack, S. (1993): Combuster Development for Advanced Helicopter Engines, MTU FOCUS 1/93. In the nozzle type shown by said FIG. 1, the fluid is injected in the form of thin kerosene jets through small bores against the interior wall of the nozzle and forms a fluid film there. The atomizing air flows between two adjacent fluid jets and forms a core air flow. Due to the shearing stress effect of this core air flow, the fluid film on the wall is driven toward the nozzle orifice. In turbine jet engines only a relatively low pressure ratio is available for the generation of the core air flow. Therefore, in these cases, it is nowhere near possible to reach the velocity of sound during atomization. Also, such known prefilming nozzles are not designed as Laval nozzles with convergent-divergent channel configuration. The known prefilming nozzles are in no way whatsoever suitable for use in process environments in industrial plants, for example, for flue gas cleaning.
2. Nozzles for the Atomization of Fluids Containing Solids.
In many cases the fluid is loaded with suspended matter, e.g., with large or small particles. The small particles may be suspended substances that are carried along—corresponding to the mesh size of a filter—as a residual solids load in the fluid to be atomized. Larger particles, usually having the shape of platelets, are formed by fragments detaching from wall deposits in the feed lines to the nozzle. The wall deposits may be fine particle deposits as well as deposits formed by substances that are initially still dissolved in the fluid. In the case of these applications, narrow channels or bores are avoided because they would become rapidly clogged by suspended matter and/or detached coarse particles carried along in the fluid. Furthermore, care is taken that the fluid does not already evaporate inside the nozzle to such an extent that a rapid buildup of deposits of the exhaust steam residue will occur there.
If the cross-sections for the fluid feed-line in the nozzle are too large, great difficulty exists in separating the massive fluid jet into fine droplets. This requires disproportionately much pressurized air, and the energy consumption of such nozzles is accordingly high.